Controlling Equine Influenza

A recent study confirmed that steps taken to manage an equine influenza outbreak in Japan were effective, working quickly to control the outbreak. On Aug. 15, 2007, 19 racehorses stabled at four race tracks came down with fever. Within a month, equine influenza spread swiftly to other tracks infecting 529 horses.

Public officials quickly responded by

Restricting movement among race tracks;

Introducing strict quarantine of potentially exposed horses;

Isolating all horses with fever and those that tested positive for equine influenza; and

Canceling races during the outbreak. Within a month, the worst of the outbreak was over, although sporadic cases continued for a year.

The interventions dropped the number of secondary cases from an average of 2.4 to 24.7 per horse to less than 1, according to Hiroshi Nishiura, MD, PhD, principal investigator of the Japan Science and Technology Agency in Saitama.

Japan had been free of equine influenza for 36 years after vaccination was mandated among racehorses following a 1971 outbreak. However, the vaccine is not 100% effective, Nishiura said. "Outbreaks can happen even among fully vaccinated horse populations, although this does not mean vaccination did not work at all," Nishiura said. "Vaccination can reduce the risks of symptomatic disease and severe manifestations."

In addition, the 2007 outbreak would have been much larger if the horses had not been vaccinated, he maintained. In the 1971 outbreak, which affected largely unvaccinated horses, the proportion of febrile horses ranged from 81.9-99.4% of racehorses, far more than 12.8% in 2007.

"I believe that vaccination reduced the total number of febrile cases in 2007," Nishiura said.

Horse owners are critical in preventing outbreaks, Nishiura said. If influenza is suspected, get the horse examined by a veterinarian and report the case to public health officials quickly. "Early examination and early reporting will help limit the extent of an outbreak," he said.

The study, "Potential effectiveness of public health interventions during the equine influenza outbreak in racehorse facilities in Japan, 2007," was published in March online ahead of print in Transboundary and Emerging Diseases.